Heat sinks are used to cool electrical devices, such as chips, diodes, and the like. Air, or some other fluid, flows over heat dissipation surfaces of the heat sink to cool the heat sink, and thus the electrical device. The heat transfer is mainly by way of convection.
Generally, natural convection is the cooling of a vertical hot surface in a large quiescent body of air. Lower-density air next to the vertical hot surface moves upward because of the buoyant force of higher-density cooler air farther away from the vertical surface. Movement of the air adjacent the vertical surface increases in velocity from zero at the vertical surface to a maximum velocity at a distance from the vertical surface and then decreases back to a velocity of zero as ambient surrounding conditions are reached. The temperature of the moving air decreases from the heated surface temperature to the ambient air temperature. As the temperature of the moving air approaches the ambient, the velocity at which the air moves approaches zero. No heat flows, by conduction or convection, where the velocity and temperature gradients equal zero, thus this outer edge is referred to as the boundary layer.
Forced convection, where air is blown across a heated surface, results in a maximum air velocity at the outer edge of the boundary layer. The difference in the velocity profile and the higher velocities provide more air near the heated surface and very thin boundary layers. Natural convection forces are present; however, the natural convection forces are negligible.
Forced convection may remove more heat than natural convection, but forced convection requires a device to move the air. In small electronic packages or where it is desirable to minimize the amount of energy expended to cool the electronic components, forced convection may be undesirable.
In free or natural convection, a tall heat sink tends to mix air inadequately with the surroundings resulting in the local ambient temperature around the heat sink to be hotter at the top of the heat sink as compared to the bottom of the heat sink, especially in laminar flow regimes. With reference to FIG. 1, a long, tall vertically-oriented heat sink 10 creates natural convection by heating the air around heat sink fins 12. This air then rises, creating an airflow that removes heat from the heat sink 10. For the section near the top of the heat sink, the local ambient air temperature is warmer than the air entering at the bottom of the heat sink 10. Although the heat sink is removing heat generated by electrical components 14 (FIG. 2) by natural convection, this local temperature rise near the top of the heat sink can have adverse effects on cooling, thus resulting in adverse effects on the electronic components mounted near the top of the heat sink.